1. Field of the Invention
The invention relates to an apparatus for drying a material of which an alignment film is composed in a process of fabrication of a liquid crystal display device, a method of drying the same, and a method of fabricating a liquid crystal display device, and more particularly to such an apparatus and methods both of which are capable of, when a material of which an alignment film is composed and which is comprised of organic solvent into which polyimide and/or polyamic acid is dissolved is dried, uniformly removing such organic solvent from the material.
2. Description of the Related Art
An alignment film in a liquid crystal display device, after being rubbed, aligns liquid crystal molecules in a single direction, and defines a pre-tilt angle. A pre-tilt angle is much influenced further by steps of forming an alignment film, to be carried out before the alignment film is rubbed.
FIG. 1 illustrates a conventional apparatus for forming an alignment film. Hereinbelow is explained a conventional method of forming an alignment film, with reference to FIG. 1.
The conventional apparatus 300 illustrating in FIG. 1 is comprised of a first unit 310 for coating a material of which an alignment film is composed, onto a substrate, a second unit 320 for transporting the substrate, and a third unit 330 for drying the material.
First, a transparent substrate 351 such as a glass substrate is placed on a stage 311 in the first unit 310.
A dispenser 312 contains a coating material 322 comprised of organic solvent, and soluble polyimide and/or polyamic acid dissolved in the organic solvent at 1 to 15 wt %.
The coating material 322 is dropped onto a first roll 313 from the dispenser 312. The first roll 313 cooperates with a second roll 314 to keep the coating material 322 at a uniform thickness. The coating material 322 is then transferred onto a surface of a third roll 315. Thereafter, the coating material 322 is printed onto a surface of the transparent substrate 351 at a uniform thickness.
Thus, a pre-alignment film 323 is formed on the transparent substrate 351.
The second unit 320 takes the transparent substrate 351 on which the pre-alignment film 323 is formed, out of the stage 311, and then, puts the transparent substrate 351 onto a hot plate 331 on the third unit 330.
The hot plate 331 is kept being heated at 50 to 100 degrees centigrade. The pre-alignment film 323 is heated by the hot plate 331 for about 30 seconds to about 3 minutes with the result that organic solvent contained in the pre-alignment film 323 is evaporated. The pre-alignment film 323 is thus dried, and resultingly, turned into a second pre-alignment film 333.
After having been dried, the second pre-alignment film 333 is baked at 180 to 250 degrees centigrade for 10 minutes to 3 hours.
FIG. 2A illustrates development of a dried area in the pre-alignment film 323 in the case that the pre-alignment film 323 is formed entirely over the transparent substrate 351.
As illustrated in FIG. 2A, a dried area in the pre-alignment film 323 develops towards a center from an edge of the transparent substrate 351 with the lapse of time.
Specifically, when ten seconds has passed after commencement of heating the substrate 351, a dried area in the pre-alignment film 323 develops to a line 324, and similarly, when twenty and thirty seconds have passed after commencement of heating the substrate 351, a dried area in the pre-alignment film 323 develops to lines 325 and 326, respectively.
As is readily understood in view of FIG. 2A, a dried area in the pre-alignment film 323 develops at a marginal area more rapidly than at a central area of the pre-alignment film 323.
FIG. 2B illustrates development of a dried area in the pre-alignment film 323 in the case that the pre-alignment film 323 is formed in four regions on the transparent substrate 351.
It is understood in view of FIG. 2B that a dried area in the pre-alignment film 323 develops at a marginal area more rapidly than at a central area of each of the four regions in each of which the pre-alignment film 323 is formed.
FIG. 3 illustrates another conventional method of drying the pre-alignment film 323 formed on the transparent substrate 351.
If the transparent substrate 351 is placed directly on the hot plate 331, the substrate 351 might be electrically charged or contaminants might attach to the substrate 351, when the substrate 351 is peeled off the hot plate 351. In order to prevent the substrate 351 from being electrically charged and contaminants from attaching to the substrate 351, a plurality of pins 336 are formed on the hot plate 331, and the transparent substrate 351 is supported on the pins 336, in the conventional method illustrated in FIG. 3.
In this method, since heat generated in the hot plate 331 is transferred to the pre-alignment film 323 through the pins 336, the pre-alignment film 323 is caused to dry first in an area closer to each of the pins 336.
Accordingly, if the pre-alignment film 323 is rubbed in the above-mentioned condition for making an alignment film, a pre-tilt angle defined by a surface condition of an alignment film and liquid crystal molecules would be caused different from one another in dependence on regions of the alignment film. According to the experiments having been conducted by the inventor, a pre-tilt angle scattered in the range of 4.5 to 6.0 degrees with the result of significant irregularity in displayed images.